Electron-transfer mechanisms with photoactivated quinones. The encounter complex versus the Rehm-Weller paradigm

Photoexcited quinones (Q*) are efficiently quenched by polymethylbenzenes ( ArH) via electron transfer (ET). However, the second-order rate constants ( k(2)) exhibit Rehm-Weller (outer-sphere) dependence an the free energy (Del ta G(ET)). despite our new findings that the quenching occurs via a series of rather strong encounter complexes [Q*, ArH] with substantial (charge-tra nsfer) bonding. The relatively high formation constants (K-EC) Of the encou nter complexes indicate that any mechanistic interpretation of the driving- force dependence of the observed rate constants is highly ambiguous since k (2) must be a composite of K-EC and the intrinsic rate constant (k(ET)) for electron transfer within the intermediate (inner-sphere) complex. As such, the reorganization energies extracted from Rehm-Weller plots lack thermody namic significance. On the other hand, the unambiguous driving-force depend ence of k(ET) represents a unique example For the "normal" Marcus behavior of the endergonic electron transfer between the donor/acceptor pair in van der Waals contact as extant in the encounter complex.